In a UMTS (Universal Mobile Telecommunications System) network, attempts are made to optimize features of the system, which are based on W-CDMA (Wideband Code Division Multiple Access), by adopting HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access), for the purposes of improving spectral efficiency and improving the data rates. With this UMTS network, LTE (Long-Term Evolution) is under study for the purposes of further increasing high-speed data rates, providing low delay, and so on (non-patent literature 1).
In a third-generation system, it is possible to achieve a transmission rate of maximum approximately 2 Mbps on the downlink by using a fixed band of approximately 5 MHz. Meanwhile, in an LTE system, it is possible to achieve a transmission rate of about maximum 300 Mbps on the downlink and about 75 Mbps on the uplink by using a variable band which ranges from 1.4 MHz to 20 MHz. Furthermore, with the UMTS network, successor systems of LTE are also under study for the purpose of achieving further broadbandization and higher speed (for example, LTE-advanced (“LTE-A”)). The system band of an LTE-A system includes at least one component carrier (CC), where the system band of the LTE system is one unit. Achieving broadbandization by gathering a plurality of components carriers (cells) in this way is referred to as “carrier aggregation” (CA).
In the uplink of the LTE-A system, application of SC-FDMA as a radio access scheme is under study. In order to maintain uplink single-carrier transmission characteristics even when a plurality of uplink frequency blocks are set, a study is in progress to transmit, selectively, retransmission control information (ACK/NACK and so on) for downlink data signals (PDSCH signals) transmitted individually from a plurality of downlink CCs, from a single CC (for example, from the primary cell (P-cell)). In this case, a user terminal needs to allocate PUCCH resources in order to feed back a plurality of retransmission control signals (retransmission control information) in response to the downlink data signals of each CC. In Rel. 10 LTE, a study is in progress to apply channel selection and/or the like for PUCCH resource allocation for retransmission control signals for two cells (two CCs) (non-patent literature 2).
Now, as uplink (UL) and downlink (DL) duplexing methods, there are frequency division duplexing (FDD), which divides between the uplink and the downlink based on frequency, and time division duplexing (TDD), which divides between the uplink and the downlink based on time. In Rel. 10 LTE, when carrier aggregation is executed in TDD, the ratio between uplink subframes and downlink subframes (TTIs: Transmission Time Intervals) is the same in all component carriers (see FIG. 1A). Meanwhile, in Rel. 11 LTE, a study is in progress in which, when carrier aggregation is executed in TDD, the ratio of uplink subframes and downlink subframes is changed per component carrier, taking into account application of a heterogeneous network and/or the like (see FIG. 1B).
Furthermore, a study is in progress, in which, as shown in FIG. 2, when carrier aggregation is employed, for example, downlink control information (DCI #2) for the downlink shared channel (PUSCH) that is transmitted in component carrier CC #2 (secondary cell (S-cell)) is multiplexed over the downlink control channel (PDCCH) in another component carrier CC #1 (P-cell)) and transmitted (cross-carrier scheduling). When this takes place, a DCI configuration, in which a carrier indicator (CI) is added in order to identify which component carrier's (between CC #1 and CC #2) downlink shared channel information the downlink control information (DCI #2) provides, is adopted.